The present description is directed to methods and apparatus for recovering from “glitches” of an ion beam during ion implantation of semiconductor wafers, specifically in stationary-beam ion implanters that employ a planar or so-called “ribbon” beam. A glitch is a sudden transient in the beam current that can adversely affect the dose uniformity of the implanted species on the wafer. It has been common in the past to interrupt an implantation pass or scan by extinguishing the ion beam when a glitch is detected, leaving the wafer with only a partial implantation from the pass. The beam is extinguished by de-energizing the plasma arc within the source chamber. Subsequently, after the ion beam has be re-established, a second implantation pass (called a “recovery” pass) is begun that proceeds in the opposite direction along the wafer and is terminated at exactly the location where the first pass was interrupted, such that the wafer has received a single uniform dose across its surface.
One problem with the prior glitch recovery technique is the possibility of a second glitch occurring during the recovery pass, such that a central section of the wafer has not yet received the desired dose (especially on the last pass). It has generally not been possible in ribbon-beam ion implanters to re-establish the ion beam as quickly as would be necessary to begin a third pass (second recovery pass) at precisely the point at which either the first or second pass was interrupted. In some cases, the wafer must be scrapped, or other undesirable measures employed. There are other situations in which it would be desirable to have the ability to quickly establish an ion beam so as to resume implantation at a precise location on a wafer.